1. Technical Field
This invention relates to sheet inspection stations used in the manufacture of continuous-sheet materials such as papers, plastics, foils, and the like. More particularly, it relates to the means by which the sheet of material is guided through the inspection station. Yet more particularly, it concerns sheet inspection stations and associated processes employing pneumatic means for guiding the sheet through the inspection station in non-contacting fashion.
2. Description of Background Art
Continuous-manufacturing processes employ measurement and control systems to derive material savings and enhance quality control. In some industries, notably that of papermaking, sheet inspection stations are used to continuously monitor certain physical properties of a material which is being continuously produced in sheet form. These inspection stations employ one or more measurement sensors. Hereinafter, for any given sheet inspection station, these sensors are collectively referred to as a "sensor package". The sensor package is mounted usually through a carriage or sheet-traversing mechanism on a conventional frame. The frame typically houses a drive system that enables movement of the carriage, with the sensor package, back and forth across the sheet of material so that measurement of the relevant physical properties can be made across the entire cross-machine width of the sheet.
The sensors making up the sensor package are technically sophisticated apparatus that typically employ a source of electromagnetic radiation in conjunction with a radiation detection device in order to produce electrical output signals, the magnitudes of which vary according to the degree of interaction of the radiation with one or more constituents of the sheet. These output signals are delivered to a remote signal processing unit such as a digital computer, where they are appropriately processed to produce measurements of the physical properties of interest.
In most cases, performance of the sensors is impaired if the sheet is permitted to flutter or is not kept perfectly flat in the region where the radiation interacts with the sheet. Therefore, some sensors require that in the region, the sheet be maintained against a flat surface of the sensor. Others employ air bearings within the sensor to maintain the sheet in a substantially constant vertical position as it passes through that region, while preventing contact of the sheet with the sensor. See, for example U.S. Pat. Nos. 4,319,847 Howarth and 4,449,398 Williams. To assist in meeting these requirements, and for other reasons such as prevention of sheet breakage, sheet inspection stations of the above description have heretofore incorporated sheet-contacting guide rails which guide the sheet into and out from the sensor package while maintaining slight tension on the sheet. These are typically rounded, metal rails that are located just outside the sensor package housing and are secured thereto. Although these guide rails are of considerable importance, their method of operation creates a problem that is more or less severe, depending on the type of sensor. Specifically, and especially where the sheet is composed of a fibrous material such as paper, contact of the sheet with the guide rails causes gradual removal of fiber or duct from the sheet. Some of this residual material finds its way into the sensor package and is deposited onto the sensors, thereby impairing sensor performance. A past solution to this problem has been to incorporate "air wipes" in the designs of sensors which are susceptible to such performance impairment. These serve to blow away accumulated residual material and mauy also serve the purpose of maintaining substantially constant air temperature in the region where radiation interacts with the sheet. That solution imposes a requirement that each dust-sensitive sensor contain its own air wipe and, insofar as residue from friction between the sheet and the guide rails is concerned, does not address the problem at its source.
This invention provides a sheet inspection station that eliminates measurement error resulting from fiber/dust buildup associated with friction between conventional guide rails and the sheet. In addition, the invention eliminates the need for periodic replacement of guide rails which are worn flat from continuous contact with the sheet.